Abstract
Accurate in situ leaf area index (LAI) estimates of forest plots are required to validate currently-used LAI map products. Woody-to-total area ratio ( α ) is a crucial parameter in converting the plant area index estimates of forest plots obtained by optical methods into LAI. Although optical methods for estimating the α of forest canopy have been proposed, their performance has never been assessed. In this study, five Larix gmelinii Rupr. forest plots with contrasting plot characteristics (i.e., tree age, tree height, management activities, stand density, and site conditions) were selected. The performance of two commonly used optical methods, namely, multispectral canopy imager (MCI) and digital hemispherical photography (DHP), in estimating the α of L. gmelinii forest plots was evaluated by using the reference α of the selected forest plots. The reference α of forest plots was measured via destructive method by harvesting two or three representative trees in each plot. Large variations were observed amongst the reference α of the selected forest plots (ranging from 0% to 56%). These α were also highly correlated with the site conditions and management activities in these plots. The effective α ( α e ) or α estimated using the leaf-on and leaf-off periods MCI or DHP images with or without consideration of the clumping effects of canopy element and woody components were 1.57 to 4.63 times the reference α in the five plots. The overestimation of α or α e was mainly caused by the preferential shading of woody components by the shoots in the leaf-on canopy. Accurate α estimates for the L. gmelinii forest plots with errors of less than 20% can be obtained from MCI when the clumping effects of canopy element and woody components are considered in the estimation.
Highlights
Leaf area index (LAI) is typically used in many models to describe vegetation–atmosphere interactions as crucial variable controlling processes, such as photosynthesis, respiration, and rain interception [1]
The Global Climate Observing System (GCOS) has specified that LAI map product values must be within 20% of in situ LAI measurements and be improved to within 5% for future applications [1,13]
In situ LAI measurements can be estimated based on the gap fraction or radiation attenuation measurements collected by using optical methods
Summary
Leaf area index (LAI) is typically used in many models to describe vegetation–atmosphere interactions as crucial variable controlling processes, such as photosynthesis, respiration, and rain interception [1]. In situ LAI measurements usually provide LAI estimates at the plot scale (e.g., 20 × 20–100 × 100 m). Forests 2018, 9, 746 provide LAI estimates at the regional, national and global scales. Over the past two decades, several commonly used LAI map products have been published, including CYCLOPES [4], GLOBCARBON [5], GLASS [6], MODIS [7], and GEOV1 [8]. Accurate and reliable in situ LAI measurements are needed to assess and improve the accuracy of these products and for them to match the accuracy requirements of several applications, such as the Global Climate Observing System (GCOS). The GCOS has specified that LAI map product values must be within 20% of in situ LAI measurements and be improved to within 5% for future applications [1,13]
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